Studies on the chemical reduction of polynuclear titanium(iv) nitrido complexes

Abstract

The redox chemistry of cube-type titanium(IV) nitrido complexes [{Ti₄(η⁵-C₅Me₅)₃(R)}(μ₃-N)₄] (R = η⁵-C₅Me₅ (1), N(SiMe₃)₂ (2), η⁵-C₅H₄SiMe₃ (3), and η⁵-C₅H₅ (4)) was investigated by electrochemical methods and chemical reactions. Cyclic voltammetry studies indicate that 1–4 undergo a reversible one-electron reduction at ca. −1.8 V vs. ferrocenium/ferrocene. Thus, complex 1 reacts with sodium sand in tetrahydrofuran to produce the highly reactive ionic compound [Na(thf)₆][{Ti(η⁵-C₅Me₅)}₄(μ₃-N)₄] (5). The treatment of complexes 1–4 in toluene with one equivalent of [K(C₅Me₅)] in the presence of macrocycles (L) leads to C₁₀Me₁₀ and the formation of more stable derivatives [K(L)][{Ti₄(η⁵-C₅Me₅)₃(R)}(μ₃-N)₄] (R = η⁵-C₅Me₅, L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe₃)₂, L = 18-crown-6 (8), crypt-222 (9); R = η⁵-C₅H₄SiMe₃, L = 18-crown-6 (10), crypt-222 (11); R = η⁵-C₅H₅, L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C₅Me₅)] and 18-crown-6 affords [{(18-crown-6)K}₂(μ–η⁵:η⁵-C₅H₅)][{Ti₄(η⁵-C₅Me₅)₃(η⁵-C₅H₅)}(μ₃-N)₄] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(μ–η⁵:η⁵-C₅H₅)Ti₄(η⁵-C₅Me₅)₃(μ₃-N)₄]. In contrast to the cube-type nitrido systems 1–4, the cyclic voltammogram of the trinuclear imido-nitrido titanium(IV) complex [{Ti(η⁵-C₅Me₅)(μ-NH)}₃(μ₃-N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C₅Me₅)] to afford C₅Me₅H and the diamagnetic derivative [{K(μ₄-N)(μ₃-NH)₂Ti₃(η⁵-C₅Me₅)₃(μ₃-N)}₂] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(μ₃-N)(μ₃-NH)₂Ti₃(η⁵-C₅Me₅)₃(μ₃-N)}] (L = 18-crown-6 (16), dibenzo-18-crown-6 (17)). Complex 17 further reacts with one equiv. of dibenzo-18-crown-6 to yield the ion-separated compound [K(dibenzo-18-crown-6)₂][Ti₃(η⁵-C₅Me₅)₃(μ₃-N)(μ-N)(μ-NH)₂] (18) similar to the ion pair [K(crypt-222)][Ti₃(η⁵-C₅Me₅)₃(μ₃-N)(μ-N)(μ-NH)₂] (19) obtained in the treatment of 15 with cryptand-222.